Evaluation of GPS RO derived precipitable water vapor against ground-based GPS observations over Iran

For more than two decades, the Global Positioning System (GPS) under a method called GPS meteorology, has been providing valuable products and parameters for meteorologists and climatologists in addition to its main purpose, which is positioning. GPS meteorology can be used in both space-based and ground-based modes. The space-based approach, called GPS Radio Occultation (RO), is used to provide the profiles of refractivity, temperature, pressure, and water vapor pressure in a neutral atmosphere and electron density in the ionosphere. However, ground-based GPS meteorology is utilized to estimate the tropospheric delay of the GPS signals and Precipitable Water Vapor (PWV) value. To date, GPS RO profiles have been used in several researches to study ionosphere and troposphere layers in Iran. However, no studies have yet used these data to estimate and evaluate PWV. In this study, GPS RO profiles were used to calculate and evaluate PWV over the study area. For statistical comparison, ground-based PWV (GB PWV) estimates in 41 stations in the study region have been considered reliable values. After selecting the pair of PWV values obtained from the space-based and ground-based GPS meteorology in the region, statistical parameters were extracted. In general, the results showed that the GPSRO PWV values have 80% correlation with the corresponding values obtained from the ground-based method. The average and RMSE of the GB-GPSRO PWV differences in the region were estimated at 3 mm and 5.2 mm, respectively. Also, the effective parameters on the accuracy of GPSRO PWV values such as seasonal changes, the position of stations, the difference in height of the lowest point of the GPS RO profile from the ground (dh), and the horizontal distance between the profile and the ground station were examined. The correlation of GPSRO PWV and GB PWV for winter, spring, summer, and autumn seasons were estimated at 0.75, 0.72, 0.73, and 0.85, respectively. The reason for the greater correlation between these two methods in the cold seasons of the year can be attributed to the lower variation of PWV values in these seasons. After sensitivity analysis of the factors considered in relation to the quality of GPSRO PWV values, statistical comparison between GB and GPS RO methods was performed using new conditions. The results showed that with dh <500m condition, the MBE and RMSE of GPSRO PWV compare to ground-based method decreased by about 50% and 25%, respectively, and the correlation between these two methods improved by 5%.